4th. The fœtus further differs from the adult in having an umbilical vein, which comes from the placental tissue, traverses the length of the cord, passes through the umbilical ring, is mostly distributed to the liver, but has a supplemental vein situated at the thick edge of the liver, and leading to the vena cava ascendens, called the _ductus venosis_. The general effect of all these peculiarities is to render the heart virtually a single one; to provide for the quiescent state of the lungs, and to modify the distribution of fresh blood. THE COURSE OF THE BLOOD IN THE FŒTUS IS AS FOLLOWS: The blood circulating in the umbilical vein is, on entering the fœtus, a part of it discharged through the _ductus venosis_ into the vena cava; another part is distributed to the liver, and is brought to the vena cava by the hepatic veins, and then mingles also with that from the inferior extremities, and then with that from the upper extremities as it passes into the right auricle. A part of this is transmitted through the right ventricle, and thence (except a supply for the nourishment of the lungs) through the ductus arteriosis into the descending aorta. A second and larger part passes through the foramen ovale into the left auricle, then into the left ventricle and arch of the aorta, the branches of which supply the head and upper extremities. The continued stream passes into the descending aorta, mixing with that already described. The whole now descends to the lower part of the aorta, where a portion is sent to the inferior extremities, but a larger part is drawn into the umbilical arteries, and is carried by them into the placenta. After birth remarkable changes take place. Something in the circumstances in which the child is placed stimulates respiration and crying, by which means the lungs are inflated, and space is afforded to the pulmonary circulation, which supercedes the use of the foramen ovale and ductus arteriosis; the blood from the lower extremities cannot pass through the umbilical arteries, and does pass through the ascending cava into the right auricle and ventricle, then into the lungs, where it undergoes analagous changes to those effected in the placenta, and is distributed to the body generally. The fœtal openings are generally obliterated in the course of a week, though the foramen ovale, or the ductus venosis, may continue pervious for two or three weeks; but soon the ductus venosis and the umbilical arteries are obliterated and the adult circulation established. PART III. PREGNANCY AND PARTURITION. CHAPTER I. DIAGNOSIS OF PREGNANCY.